Occurrence of sulfur-driven autotrophic denitrification in demonstration-scale treatment wetlands.

摘要

Four demonstration-scale constructed treatment wetlands receiving advanced-secondary treated wastewater were investigated over the time period 1997 through 2000. In general terms, the research goal was to determine if evidence exists to document the occurrence of sulfur-driven autotrophic denitrification in demonstration-scale treatment wetlands. Efforts focused upon characterizing vegetative cover so that macrophytic-based carbon inputs could be developed. Hydraulic and material mass balances were developed for oxygen, total organic carbon, organic nitrogen, ammonia, nitrite, nitrate, and sulfate. Denitrification rates based on mass balance results were found to be independent of percent vegetative cover. Data also suggested that increased nitrate removal may be a function of increasing infiltration rates; however, statistically, this was confounded with increasing hydraulic loading rates. Mass balances were also used to develop stoichiometric ratios of sulfate production to nitrate removal which, on several occasions, fell within the range of 0.625 to 1.62 (molar), suggesting that sulfur-driven autotrophic denitrification was occurring in the wetland basins studied. A biogeochemical model was then developed to assess theoretical carbon utilization within the wetlands. Under low, average, and high macrophytic-based carbon inputs, over-utilization of organic carbon occurred in at least one wetland. Since field-derived annual mass removals of oxygen, nitrogen, and sulfur were not supported by the available carbon, an additional pathway for NO_x -N was indicated, namely sulfur-driven autotrophic denitrification. Under this scenario, carbon-reducing equivalents were stored in the wetland sediments as reduced sulfur species. In times of competition or when there is insufficient labile carbon, bacteria use the reduced sulfur species as electron donors for nitrate reduction. Such a mechanism has not been reported in the literature to date and can be used to explain the occurrence of areal NO_x-N removal rates that are independent of wetland percent vegetative cover.